An orthopedic screw having a thread with two parts, a distal and a proximal part, each having a different thread configuration. The distal section has a thread with outer dimension and pitch suitable for entry into cancellous bone, while the proximal section has a composite thread comprising (i) a first thread of the same or slightly larger outer diameter as the cancellous thread in the distal section, having the same pitch thereof, and lying on the same helix, and (ii) another thread having a smaller outer diameter but the same pitch as the first thread, but disposed on a helix displaced from that of the first thread, such that it lies between the crests of the first thread. This screw enables optimum fixation strength in a bone or bones having a harder cortical outer section and a softer cancellous inner section. The screw may have an unthreaded central section.